It is well known to include in various kinds of imaging elements, a transparent layer containing magnetic particles dispersed in a polymeric binder. The inclusion and use of such transparent magnetic recording layers in light-sensitive silver halide photographic elements has been described in U.S. Pat. Nos. 3,782,947; 4,279,945; 4,302,523; 4,990,276; 5,215,874; 5,217,804; 5,229,259; 5,252,441; 5,254,449; 5,335,589; 5,395,743; 5,413,900; 5,427,900; 5,498,512; 5,709,984 and others. Such elements are advantageous because images can be recorded by customary photographic processes while information can be recorded simultaneously into or read from the magnetic recording layer by techniques similar to those employed for traditional magnetic recording art.
The transparent magnetic recording layer must be capable of accurate recording and playback of digitally encoded information repeatedly on demand by various devices such as a camera or a photofinishing or printing system. The magnetic layer also must exhibit excellent running, durability (i.e., abrasion and scratch resistance), and magnetic head-cleaning properties without adversely affecting the imaging quality of the photographic elements. However, this goal is extremely difficult to achieve because of the nature and concentration of the magnetic particles required to provide sufficient signal to write and read magnetically stored data and the effect of any noticeable color, haze or grain associated with the magnetic layer on the optical density and granularity of the photographic layers. These goals are particularly difficult to achieve when magnetically recorded information is stored and read from the photographic image area. Further, because of the curl of the photographic element the magnetic layer must be held more tightly against the magnetic heads than in conventional magnetic recording in order to maintain planarity at the head-media interface during recording and playback operations. Thus, all of these various characteristics must be considered both independently and cumulatively in order to arrive at a commercially viable photographic element containing a transparent magnetic recording layer that will not have a detrimental effect on the photographic imaging performance and still withstand repeated and numerous read-write operations by a magnetic head.
Problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper have been recognized for many years by the photographic industry. The accumulation of charge on film surfaces leads to the attraction of dust, which can produce physical defects. The discharge of accumulated charge during or after application of the sensitized emulsion layers can produce irregular fog patterns or static marks in the emulsion. The severity of the static problems has been exacerbated greatly by increases in sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process results primarily from the tendency of webs of high dielectric constant polymeric film base to undergo triboelectric charging during winding and unwinding operations, during transport through the coating machines, and during post-coating operations such as slitting, perforating, and spooling. Static charge can also be generated during the use of the finished photographic product. In an automatic camera, because of the repeated motion of the photographic film in and out of the film cassette, there is the added problem of the generation of electrostatic charge by movement of the film across the magnetic heads and by repeated winding and unwinding operations, especially in a low relative humidity environment. The accumulation of charge on the film surface results in the attraction and adhesion of dust to the film. The presence of dust not only can result in the introduction of physical defects and the degradation of the image quality of the photographic element but also can result in the introduction of noise and the degradation of magnetic recording performance. This degradation of magnetic recording performance can arise from various sources including signal loss resulting from increased head-media spacing, electrical noise caused by discharge of the static charge by the magnetic head during playback, uneven film transport across the magnetic heads, clogging of the magnetic head gap, and excessive wear of the magnetic heads. In order to prevent these problems arising from electrostatic charging, there are various well known methods by which an electrically-conductive or antistatic layer can be introduced into the photographic element to dissipate any accumulated electrostatic charge.
The use of such electrically-conductive layers containing suitable semi-conductive metal oxide particles dispersed in a film-forming binder in combination with a transparent magnetic recording layer in silver halide imaging elements has been described in the following examples of the prior art. Photographic elements comprising a transparent magnetic recording layer and a transparent electrically-conductive layer both located on the backside of the film base have been described in U.S. Pat. Nos. 5,147,768; 5,229,259; 5,294,525; 5,336,589; 5,382,494; 5,413,900; 5,457,013; 5,459,021; and others. The conductive layers described in these patents contain fine granular particles of a semi-conductive crystalline metal oxide such as zinc oxide, titania, tin oxide, alumina, indium oxide, silica, complex or compound oxides thereof, and zinc antimonate or indium antimonate dispersed in a polymeric film-forming binder.
Antistatic backing or subbing layers containing colloidal "amorphous" vanadium pentoxide, especially silver-doped vanadium pentoxide, as described in U.S. Pat. Nos. 4,203,769 and 5,439,785, are highly effective at providing static protection, have excellent transparency and are not significantly dependent on humidity. Colloidal vanadium pentoxide is composed of entangled microscopic fibrils or ribbons 0.005-0.01 .mu.m wide, about 0.001 .mu.m thick, and 0.1-1 .mu.m in length. However, colloidal vanadium pentoxide is soluble at the high pH typical of developer solutions for photographic processing and must be protected by a nonpermeable barrier layer. Examples of suitable barrier layers are taught in U.S. Pat. Nos. 5,006,451; 5,221,598; 5,284,714; and 5,366,855, for example. Further, when a conductive layer containing colloidal vanadium pentoxide underlies a transparent magnetic layer, the magnetic layer inherently can serve as a nonpermeable barrier layer. However, if the magnetic layer contains a high level of reinforcing filler particles, such as .gamma.-aluminum oxide or silica fine particles, it must be crosslinked using suitable cross-linking agents in order to preserve the desired barrier properties, as taught in U.S. Pat. No. 5,432,050.
Alternatively, a film-forming sulfopolyester latex or polyesterionomer binder can be combined with the colloidal vanadium pentoxide in the conductive layer to minimize degradation during wet processing as taught in U.S. Pat. Nos. 5,360,706; 5,380,584; 5,427,835; 5,576,163; and others. Furthermore, it is disclosed that the use of a polyesterionomer can improve solution stability of colloidal vanadium pentoxide containing dispersions. Instability of vanadium pentoxide gels in the presence of various binders is well known and several specific classes of polymeric binders have been identified for improved stability or coatability, for example in U.S. Pat. Nos. 5,427,835; 5,439,785; 5,360,706; and 5,709,984. U.S. Pat. No. 5,427,835 teaches the use of sulfopolymers in combinations with vanadium oxide preferably prepared from hydrolysis of oxoalkoxides for antistatic applications. A specific advantage cited for preparation of vanadium oxide gels from oxoalkoxides is the ability to control the vanadium oxidation state. Colloidal vanadium oxide gels are described as viscous dark brown solutions which become homogeneous upon aging. Comparative Example 3 describes the formation of "dark greenish clots" upon mixing with polyacrylic acid indicating a change in oxidation state and flocculation of the gel. Similarly, the examples of sulfopolymers with vanadium oxide result in a color change from dark brown to dark greenish-brown, again indicating a potentially undesirable change in vanadium oxidation state.
U.S. Pat. No. 5,439,785 teaches the use of a specified ratio of sulfopolymer to vanadium oxide to provide an antistatic formulation which remains conductive after photographic processing. A range of from 1:20 to 1:150 V.sub.2 O.sub.5 :sulfopolymer is specified. Surface electrical resistivity values are typically greater than 1.times.10.sup.9 ohm/square for the indicated range. At lower colloidal vanadium oxide concentrations, the conductivity is insufficient to provide antistatic protection; at higher vanadium oxide concentrations the antistatic layer loses conductivity when subjected to photographic processing. However, prior art colloidal vanadium pentoxide typically have significantly lower resistivity values, i.e., 1.times.10.sup.8 ohm/square. Consequently, one of the primary benefits of colloidal vanadium oxide, low resistivity at low dry weight coverage is not achieved.
Colloidal vanadium oxide dispersed with a terpolymer of vinylidene chloride, acrylonitrile, and acrylic acid coated on subbed polyester supports and overcoated with a transparent magnetic recording layer is taught in U.S. Pat. Nos. 5,432,050 and 5,514,528. U.S. Pat. No. 5,514,528 also teaches an antistatic layer consisting of colloidal vanadium oxide and an aqueous dispersible polyester coated on a subbed polyester support and subsequently overcoated with a transparent magnetic recording layer.
U.S. Pat. No. 5,718,995 teaches an antistatic layer containing colloidal vanadium oxide and a specified polyurethane binder having excellent adhesion to surface treated polyester supports and an overlying transparent magnetic layer. However, it is further disclosed that the coating composition has limited shelf-life (less then 48 hrs.). In order to overcome the limited shelf life, a mixed melt process was preferably used in which separate solutions of colloidal vanadium pentoxide and of the polyurethane binder were prepared and mixed in-line just prior to the coating hopper. This results in an undesirable complication of the coating process. It is further disclosed in '995 that it is difficult to achieve adequate adhesion to glow discharge treated polyethylene naphthalate for a magnetics backing package consisting of a solvent coated cellulosic-based magnetic layer overlying an antistatic layer containing colloidal vanadium pentoxide and the preferred sulfopolyesters or interpolymers of vinylidene chloride cited in the above mentioned U.S. patents.
In addition to the aqueous-based coating compositions described above it may be advantageous to coat antistatic layers from solvent-based formulations. U.S. Pat. No. 5,709,984 describes antistatic layers containing colloidal vanadium oxide gel, a volatile aromatic compound, and a polymeric binder prepared from a solvent-based dispersion using acetone and ethanol. Polymeric binders demonstrated include interpolymers of vinylidene chloride, polymethylmethacrylate, cellulose nitrate and cellulose diacetate. It is further disclosed that due to the exceptional adhesion requirements of antistatic layers containing colloidal vanadium oxide, such layers generally exhibit poor adhesion when directly coated on untreated or unsubbed supports, especially when overcoated with a transparent magnetic recording layer. Furthermore, it is particularly difficult to achieve adequate adhesion for a cellulosic-based transparent magnetic recording layer, especially when the polymeric binder/vanadium oxide gel ratio is less than 1/1.
U.S. Pat. No. 5,455,153 describes photographic elements containing a clad vanadium pentoxide layer. The cladding layer is formed by applying an overcoat of an oxidatively polymerizable compound which may be applied neat to the vanadium oxide or in the form of an aqueous solution, a solvent solution or as a vapor. Suitable oxidatively polymerizable monomers include anilines, pyrroles, thiophenes, furans, selenophenes and tellurophenes. Antistatic layers containing clad vanadium oxide were demonstrated to have improved resistance to basic solutions as typically encountered during conventional photographic processing. Improved base resistance results from cladding the surface of vanadium pentoxide rather than a change resulting from polymer intercalation between vanadium oxide layers.
Intercalation of various species, including cations, metal-containing complexes, organic molecules and polymers, within the vanadium oxide gel structure is well-known, particularly in the catalysis field and as cathode materials for batteries. However, intercalated colloidal vanadium oxide for antistatic applications has not typically been addressed.
U.S. Pat. No. 5,659,034 describes intercalation of metal coordination complexes, particularly Zn(2,240 -dipyridyl).sub.2, between layers of vanadium oxide. The resultant intercalated vanadium oxide was described as black rod-shaped crystals which are unsuitable for antistatic applications for photographic films.
U.S. Pat. No. 5,073,360 describes the formation of bridged/lamellar metallic oxides having intercalated spheroidal cationic species. The preferred metallic oxide is vanadium pentoxide and the spheroidal cationic species is preferably an aluminum polyoxocation, particularly [Al.sub.13 O.sub.4 (OH).sub.24].sub.7.sup.+. The vanadium oxide gel can be prepared for example by ion exchange or melt quenching. The intercalated material is then isolated by filtration, dried and optionally calcined to give high surface area materials which are particularly suited as molecular sieve filters, catalysts, and catalyst supports. However, no indication is given regarding the antistatic properties of the intercalated vanadium oxide.
Intercalation of a wide variety of organic or polymeric materials between vanadium oxide layers in vanadium oxide gels is well known. Intercalative polymerization of aniline resulting in polyaniline is described in Mater. Res. Soc. Symp. Proc. V. 233, pp. 183-194, 1991 and Chem. Mater. V. 8, pp. 1992-2004, 1996. A significant decrease in oxygen concentration and a color change from red to dark blue was observed when vanadium oxide gel was added to an air saturated solution of aniline in water. Conductivity of the polyaniline-vanadium oxide material increased substantially upon aging. It was proposed that conductivity in the fresh material occurred by electron transport through the vanadium oxide framework (semiconductive) but upon aging a metallic-like conductivity dominated as polyaniline chains formed.
Poly(ethylene oxide) intercalated vanadium oxide gels were reported in Chem. Mater, Vol. 3, 992-994, 1991 and Chem. Mater, Vol. 8, 525-534, 1996 to be highly light sensitive, turning dark blue within several weeks for exposure to room light or within several hours for exposure to UV irradiation. Non-intercalated vanadium oxide gels were not light sensitive. In addition to a color change, the conductivity increased and solubility decreased with increasing irradiation. However, the irradiated conductivity decreased with increasing polyethylene oxide intercalation. Changes in the vanadium oxide interlayer distance due to intercalation of poly(vinylpyrrolidone) (PVP), poly(propylene-glycol) (PPG), and methylcellulose are described in Adv. Mater, Vol. 5, 369-372, 1993. Interlayer distance increased linearly for (PVP).sub.x V.sub.2 O.sub.5.nH.sub.2 O for values of x up to 3. Furthermore, a change in the chemical nature of PVP was noted and ascribed to formation of hydrogen bonding with co-intercalated water. The interlayer spacing did not vary linearly with either PPG or methylcellulose. The interlayer distance remained constant for (PPG).sub.x V.sub.2 O.sub.5.nH.sub.2 O with x values greater than 1, and PPG remained chemically unaltered. Particularly in the case of PPG, the samples were light sensitive as indicated above.
The above references indicate a vast array of organic or polymeric species can be intercalated within vanadium oxide gel structures. However, the intercalated material is frequently light sensitive and conductivity changes during aging. Furthermore, intercalation and subsequent reaction frequently decreases solubility of the vanadium oxide gel. Consequently, it would be neither anticipated nor expected that intercalation of vanadium oxide gels with water-soluble polymeric species would result in a vanadium oxide gel having improved solution stability and reduced impact of solution aging on conductivity.
The use of polyvinylpyrrolidone in antistatic formulations is also well known. For example, U.S. Pat. Nos. 4,418,141; 4,495,276; 5,368,995; 5,484,694; 5,453,350; 5,514,528 and others include polyvinylpyrrolidone amongst an extensive list of suitable binders for antistatic materials such as tin oxide or zinc antimonate. There is no specific mention or claim to enhanced properties or stability of polyvinylpyrrolidone or other water soluble vinyl-containing polymers relative to other polymeric binders for the above mentioned patents.
U.S. Pat. No. 4,489,152 describes a diffusion transfer film having an opaque layer consisting of carbon black having 2-10 percent polyvinylpyrrolidone based on the weight of carbon black. The addition of polyvinylpyrrolidone having a molecular weight of about 10,000 to the carbon black layer was found to improve the silver transfer process. However, there was no indication of antistatic properties nor of improved formulation stability for the carbon black layer.
U.S. Pat. No. 4,860,754 describes an electrically conductive adhesive material consisting of a low molecular weight plasticizer, a high molecular weight water soluble, crosslinkable polymer, uncrosslinked polyvinylpyrrolidone, and an electrolyte. The uncrosslinked polyvinylpyrrolidone is added as a tackifier. Antistatic properties of the adhesive material are insufficient for photographic applications since the electrolyte can be removed during wet photographic processing. Furthermore, ionic conductors are generally not effective when overcoated with a hydrophobic layer such as a typical transparent magnetic recording layer.
U.S. Pat. No. 5,637,368 describes the use of colloidal dispersions of vanadium oxide for imparting antistatic properties to adhesive tapes. Polyvinylpyrrolidone and polyvinylpyrrolidone copolymers are included in a list of suitable adhesive compounds. The use of vanadium oxide in the adhesive layer is suggested, but all examples consist of a separate vanadium oxide layer and a separate adhesive layer. In addition polyvinylpyrrolidone was neither demonstrated nor disclosed to give superior performance. Furthermore, use of the adhesive material having antistatic properties for use in photographic imaging applications is not suggested.
As disclosed in the above mentioned U.S. patents several polymers, for example interpolymers of vinylidene chloride, sulfopolyesters, polyesterionomers, and cellulosics have been used as binders for antistatic layers containing colloidal vanadium oxide. However, due to the solution chemistry and oxidative potential of vanadium oxide, the selection of compatible binders or a suitable vanadium oxide to binder formulation range is limited. For example, for low coating coverages vanadium pentoxide may typically be coated at 0.05 weight percent or less. Such low concentrations result in coating formulations which are prone to instability and flocculation of the vanadium oxide gel. This creates serious difficulties in accumulation of flocculated vanadium oxide plugging solution delivery lines, filters and coating hoppers. Furthermore, flocculation can result in coating defects or "slugs" which can result in optical and electrical non-uniformities in the coating. The addition of surfactants to the coating solution may stabilize the vanadium oxide gel, however, the typically high levels of surfactant required are undesirable for adhesion and coatability of subsequently applied layers, particularly magnetic recording layers. The concern of stability has been addressed in many of the above U.S. patents. Furthermore, interaction between colloidal vanadium oxide and polymeric binders can result in limited dispersion shelf-life. In addition to the potential for incompatibility of binders, it is well known that vanadium pentoxide can act as a reactant or catalyst for decomposition of organic solvents. Decomposition products can degrade the coating quality of the antistatic layer or subsequently coated layers and can adversely impact the sensitometric performance of photographic emulsions thereby requiring careful selection of coating solvents and binders for the antistatic layer or overlying layers. The indicated problems with regards to solution stability, incompatibility and potential interactions for an antistatic layer containing colloidal vanadium oxide limits the selection of possible polymeric binders which may be desired for certain physical performance requirements such as adhesion or abrasion resistance.
Because the requirements for an electrically-conductive layer to be useful in an imaging element are extremely demanding, the art has long sought to develop improved conductive layers exhibiting a balance of the necessary chemical, physical, optical, and electrical properties. As indicated hereinabove, the prior art for providing electrically-conductive layers useful for imaging elements is extensive and a wide variety of suitable electroconductive materials have been disclosed. However, there is still a critical need in the art for improved conductive layers which can be used in a wide variety of imaging elements, which can be manufactured at a reasonable cost, which are resistant to the effects of humidity change, which are durable and abrasion-resistant, which do not exhibit adverse sensitometric or photographic effects, which exhibit acceptable adhesion to overlying or underlying layers, which exhibit suitable cohesion, which have improved solution stability, which have improved binder compatibility, and which have low catalytic or reactant activity. Further, to provide both effective magnetic recording properties and effective electrical-conductivity characteristics in an imaging element, without impairing its imaging characteristics, poses a considerably greater technical challenge.
In particular, an improved colloidal vanadium oxide which is compatible with a wider selection of polymeric binders or facilitates the use of higher binder:vanadium oxide ratios to improve adhesion to the support and underlying or overlying layers is desired. It is toward the objective of providing a useful combination of a transparent magnetic recording layer and electrically-conductive layers containing colloidal vanadium oxide that more effectively meet the diverse needs of imaging elements, especially those of silver halide photographic films, but also of a wide variety of other types of imaging elements than those of the prior art that the present invention is directed.